MUSEUM OF COMFAEATIVE ZOOLOGY. 183 



find that this relation is a primitive one in the chick. From lack of 

 material, I was unable to study the younger stages of development in 

 the salmon ; but with regard to the chick, I can say definitely that at 

 its first appearance the fifth nerve does not arise from the outward con- 

 vexity of the first neuromere of the medulla, as Beraneck has repre- 

 sented it. It arises from the concavity that lies between the first and 

 second neuromeres, and its position at the side of the anterior neuro- 

 mere is quite secondary. This fact supports Marshall's theory, that 

 segmental nerves '' at an early period shift downwards, and acquire new 

 or secondary roots of attachment to the side of the brain." ^ It has 

 seemed to me that the secondary attachment of the fifth nerve was not 

 due to a new outgrowth connecting the nerve root and brain, but rather 

 to a gradual shifting of the position of the original root. However this 

 may be, it certainly cannot be owing to the outward spreading of the 

 walls of the fourth ventricle that the root of the fifth nerve comes to 

 acquire an attachment anterior to its primitive attachment. Beraneck 

 says: "Par suite des changements survenus dans la route du cei"veau 

 posterieur, . . . les replis sont encore plus dejetes sur les cotes de la 

 region cephalique et les racines nerveuses paraissent ^tre descendues et 

 se rattacher plus pres de la region ventrale qu'auparavant. . . . Les 

 changements survenus dans la forme et la structure du cerveau poste- 

 rieur me semblent suffire a expliquer cette descente apparente des nerfs 

 craniens sans qu'il faille recourir a une hypothese ne reposant sur 

 aucune observation directe." ^ The mere divergence of the walls of the 

 fourth ventricle may suffice to explain the more ventral attachment of 

 the fifth nerve, but is hardly sufficient to account for the fact that its 

 secondary attachment is anterior to the first. 



Since the walls of the fourth ventricle follow quite closely the curvet 

 on the outward surface of the medulla, the concavity from which the 

 fifth nerve arises is represented on the inside by a ridge pi'ojecting far 

 mto the fourth ventricle. A series of horizontal sections through the 

 root of the fifth nerve at the time of its attachment between the first 

 and second neuromeres shows this ridge to be composed of lines of cells 

 converging like the rays of a fan towards the point from which the 

 fifth nerve takes its origin (Fig. 10). This convergent arrangement of 

 lines of cells is characteristic of each of the ridges which, projecting- into 

 the fourth ventricle, separate successive neuromeres (Fig. 12 b). If 



1 A. Milnes Marshall. Segmental Value of the Cranial Nerves. Journal of 

 Anatomy and Physiology, 1882. 



2 Replis Me'dullaires du Poulet, Rec. zool. buiss'e, pp. 334, 335. 



